The present invention relates to a method and apparatus for the safe starting of a large induction motor and more particularly to a method and apparatus capable of accurately calculating the varying, slip dependent, rotor resistance of a motor by means of a microprocessor based measurement system.
Electric motors generate heat and require protection from overheating. The need for thermal protection is particularly acute in large motors, for example induction motors of several thousand horse power which are quite expensive and difficult and time consuming to repair.
Motors are designed to withstand internally generated heat arising from currents at rated loads. But the starting or restarting of a large induction motor or a locked rotor condition in an induction motor requires special attention since, in all of the above mentioned cases, the current flowing in the motor is significantly higher than rated load current. Since heat is generated in proportion to the square of the current, the motor can be damaged from overheating within a matter of seconds. Motors are, therefore, provided with so called relay protection devices for tripping the motor before it reaches a damaging temperature. Usually, the motor is tripped at a predetermined time following the detection of the locked rotor condition or after starting if the current or speed of the motor has not reached its operational level within a prescribed time period.
The locked rotor condition does not present a particularly difficult problem because the locked rotor current is relatively constant and so is the resistance of the rotor winding during a locked rotor condition. It is therefore a simple matter to calculate and integrate the I.sup.2 R heating effect in the motor and to trip the motor before it reaches its critical temperature.
But calculating the heating effect resulting from starting current is not as simple. The magnitude of the starting current is about equal to the locked rotor current but has a lesser heating effect. This is because the effective resistance of the rotor winding changes with slip. The rotor resistance has a high initial value at start up, when the rotor is effectively locked and the slip is by definition unity, and a final rotor resistance value which may be as little as one third of the initial resistance as the motor reaches its rated slip. The same is therefore true of the I.sup.2 R heating effect. Unfortunately, conventional protection systems respond only to the current or to speed of the motor and assume that the resistance of the rotor remains at its initial high value throughout the starting cycle.
Very large motors have larger torque and/or moment of inertia and a proportionally increased motor starting time. The starting/acceleration period of induction motors with high inertia drives approaches and may in fact exceed the time interval at the conclusion of which the motor is normally tripped by the locked rotor protection mechanism.
Several conventional protection schemes are available for dealing with the problem but they are exceedingly complex. Other simpler conventional schemes resort to the use of a so-called "under impedance relay" which defeats the locked rotor protection mechanism during the later phase of a starting cycle to permit the motor to start. It is frustrating to realize that an under impedance relay must be used to terminate protection at the point where it is needed, solely because of an apparent inability to determine the profile of the rotor resistance throughout the starting sequence.